The present invention relates in general to detecting the character of a medium introduced in a sample chamber using acoustic signals produced by volumetric absorption of radiation and more particularly concerns novel apparatus and techniques for detecting atmospheric pollutants using acoustic signals produced by volumetric absorption of infrared radiation to achieve exceptionally high acoustic amplification and thereby facilitate detection.
It is known to measure trace gaseous constituents using acoustic signals produced by volumetric absorption of infrared radiation. Such a technique is described in an article by L. B. Kreuzer in J. APPL. PHYS., 42, 2934 (1971). A laser or other source of radiant energy having a wavelength coincident with an absorption line of the species to be detected energizes a cell containing the sample mixture. The sample mixture absorbs radiation and converts it into thermal motion of the gas by intermolecular collisions. These collisions produce a pressure rise which may be detected by a sensitive microphone. The acoustic signal produced by the pressure rise is proportional to the concentration of the absorbing species. By modulating the beam of radiation at an audible rate, an easily detected periodic pressure signal is provided.
According to publications by Kreuzer, C.K.N. Patell and N.D. Kenyon in SCIENCE, 173, 45 (1971) and SCIENCE, 177, 347 (1972), this prior art approach yields detection sensitivities of a few parts per billion (ppb) for a number of important pollutants using selected gas laser wavelengths or a wavelength-tunable infrared spin-flip Raman laser as the excitation source. These publications reported encountering problems caused by background signal arising from spurious scattering and absorption by the optical windows and surfaces of the cell corresponding to a species concentration of 50-100 ppb, requiring a measurement accuracy of about 1% in the microphone signal amplitude to achieve the potential sensitivity of the apparatus.
A report dated Nov. 30, 1967 available from the Defense Documentation Center under AD 665674 at pages 32-44 suggests pulsing a laser beam along the axis of a cylindrical cavity at a rate corresponding to the cavity resonant frequency. The report does not tell how to do it in practice and states that success depends not only on the construction of a powerful laser with appropriate pulsing mechanism, but also on the construction of a highly sensitive acoustic detector coupled with the proper mode of many modes of oscillation of a cylinder.
Accordingly, it is an important object of this invention to provide improved methods and means for detecting the concentration of certain substances present in exceptionally small quantities in a medium being sampled.
It is another object of the invention to achieve the preceding object for detecting pollutants.
It is another object of the invention to achieve one or more of the preceding objects by exciting standing waves in an acoustic resonance chamber with essentially monochromatic energy modulated at a rate corresponding to the acoustic resonance of the chamber to provide high effective amplification facilitating the detection of substances in exceptionally small concentrations.
It is a further object of the invention to achieve one or more of the preceding objects with methods and means for maintaining the high gain despite changes in the resonant frequency of the acoustic chamber.